Signal detection and measurements play an important role in the application of Hearing Instruments. Among other things, they allow us to collect information about the different acoustic environments in which a Hearing Instrument is worn, to assess Hearing Instrument performance, to collect the data needed for user-specific Hearing Instrument adjustments and to verify that the Hearing Instrument operates properly after a repair.
Sometimes the Hearing Instrument itself can carry out all, or part of a measurement procedure. Using the Hearing Instrument, rather than an external device, to perform a measurement often brings significant benefits, as in the case of measuring the so-called individual threshold of feedback (also called “Critical Gain”). The individual threshold of feedback is a measure of the gain limitations that should be taken into account in order to reduce unwanted whistling sounds, and this threshold is unique for every hearing instrument fitting.
In existing solutions for measuring the individual threshold of feedback, an acoustic test signal is picked up by the Hearing Instrument's microphone and fed directly into a level meter or similar device (cf. e.g. M. Bertges-Reber, Boundaries of real open fittings: Clinical experiences, Hearing Review, Vol. 13, No. 2, February 2006, page 44-47). Such procedures are inaccurate in the presence of background noise. It is almost impossible to eliminate background noise in all cases because these measurements must be carried out while the Hearing Instrument is being worn. There are two main reasons for the inaccuracy of such procedures, both of which are related to the ratio between the signal to be measured and the unwanted background noise (signal-to-noise ratio):                Background noise can be very loud, resulting in a poor signal-to-noise ratio.        In order to avoid signals being uncomfortably loud for the Hearing Instrument wearer it may be necessary to limit the output level of the acoustic test signal. This also compromises the signal-to-noise ratio.        
The present invention addresses both of the above potential causes of inaccuracy.